import sys

def tracer(frame, event, arg):
    if event == 'line':
        if frame.f_locals['cmd'] == 'delete':
            del frame.f_trace
        elif frame.f_locals['cmd'] == 'set':
            frame.f_trace = None
        print('f_trace:', frame.f_trace)
        return None
    return tracer

def foo(cmd):
    print(cmd, 'start')
    print(cmd, 'done')

sys.settrace(tracer)
foo('delete')
foo('set')